since 1993, federal courts have overturned 46 death sentences in California on various grounds...

since 1993, federal courts have overturned 46 death sentences in California on various grounds, most of which no longer would qualify for federal review under sweeping new rules now being considered by Congress that would prohibit most federal appeals unless there is firm proof of innocence.
Here is a breakdown of reasons federal courts reversed those 46 California sentences (note that some cases involved multiple grounds):
Incompetent legal defense: 26
Errors by trial judge: 17
Police or prosecutor misconduct: 9
Jury misconduct: 3